In surveying and building construction it is often necessary to establish a datum plane which is a predetermined height above the existing terrain or, for instance, the floor of a building. Once this datum plane is established measurements can be made from this datum plane to accurately position building structures or, in the case of excavation, to establish a reference plane for a grading operation. Moreover it is sometimes useful to establish a vertical datum plane at a given location.
In the past, the datum plane has been established with the use of a transit and level rod which requires personnel both at the transit and at the level rod. As will be appreciated communication must be established between the two people manning respectively the transit and the level rod. This is, of course, a time consuming process which has to some extent been alleviated by the use of rotating lasers, the beams of which sweep an area to establish the datum plane. However, in addition to the expense of the laser system and the mechanical stability of the rotating laser, a serious problem is finding the laser beam at a point removed from the laser. When the laser is utilized in a darkened room this may be easily established. When the laser is used out of doors, establishing the position of the laser beam is difficult at the permitted laser output levels due to background radiation. If the output power of the laser could be increased without damage to the human eye, then location of the sweeping laser beam would become less difficult. However, no inexpensive "eye-safe" yet highly visible laser presently exits.
The problem of establishing a datum plane over a wide area is solved in the present invention without the use of lasers. As discussed in connection with U.S. patent application Ser. No. 492,633 filed July 29, 1974 by Philip Johnson and Richard Northrup, hereby incorporated by reference, the projection of an image having a straight boundary region by use of a flash lamp and projecting optics provides an image which can be detected at two spaced points, and apparatus is provided for utilizing the signals from two spaced detectors at these points to determine the center of the boundary region extremely accurately. This establishes the datum plane set up by the projector. The detection of the center of the boundary region, in one embodiment, is accomplished by maintaining one of the detectors within the illuminated region of the image while moving the other detector across the boundary region until such time as the level from the detector moved across the boundary region is 1/2 that of the detector which is in the illuminated region. In this manner, although the boundary region may be somewhat ill-defined, its center is very accurately defined by the position of the detector at the time that the ratio between the two detectors is 1/2. As described in the aforementioned patent application, accuracies of .+-.1/8 of an inch at 1,000 feet are achieved. Moreover, the system is unaffected by range and source intensity since the detector in the illuminated region is used as a reference level detector, to which the output of the other detector is compared. In some cases where reduced accuracy is acceptible, such as .+-.1/2 inch at 500 feet, determinations of the datum plane may be made visually, without a receiver. This is accomplished by the observer moving his head until the light flashes from the projector start to go out.
While the projector illustrated in connection with the above mentioned patent application provides for adequate image projection, it is desirable to provide a wider angle of illumination to prevent the necessity of moving the projector to accomodate various positions. While wide angle lenses may be provided for the required coverage, wide angle lenses in general suffer from both cost and limitations on the aperture size. It will be appreciated that when the subject system is utilzed out of doors, in order to have an effective range of over 1,000 feet it is necessary that as much of the light from the source, in one instance a xenon flash lamp, be utilized as possible. The limitation on aperture size for wide angle lenses in some instances precludes their use over such long distances.
Wide angle projection of an image is solved in the present invention by providing a line source and cylindrical reflecting optics with ends bounded by reflecting walls. Cylindrical reflective optics, as opposed to cylindrical refractive optics, are used because of the non-linearity of refractive optics, while reflecting optics are linear. Interposed in the path between cylindrical reflectors is a reticle which may either take the form of a non-reflective occluding knife edge or a knife edge which also includes a reflective portion. By use of the cylindrical optics almost the entire portion of the line source is focused at infinity along with the image provided via the reticle, and this takes place over beam angles exceeding 90.degree. and in some cases as much as 120.degree.. Thus the beam width of the projector may be as much as 120.degree. and rotating of the projector will be unnecessary to provide the necessary datum plane.
However, if 360.degree. coverage is required, because of the mechanically stable projector package to be described, the entire package may be spun about a vertical axis while still maintaining the accuracy required. The projector is exceedingly simple, and the requirements, unlike those for the operation of a laser, are easily met both in the stationary and rotating cases.
In an additional embodiment, the subject invention encompasses a reflecting reticle in which the reflecting surface is contoured for maximum utilization of the flash lamp source. In a still further embodiment, a non-obscurring reticle is utilized with cylindrical reflecting optics having either a parabolic or a circular cross section. In the latter case, the cylindrical main projection reflector is tilted to alleviate any aberrations caused by the circular cross section. In a still further embodiment, in order to alleviate any visual ambiguity as to the boundary region intended, color coding is utilized to identify an irregular diffuse boundary region which should be avoided.
It will be appreciated that the subject projector is primarily intended to be utilized in connection with the aforementioned boundary region detecting apparatus described in the aforementioned patent application. Since this apparatus determines the center of the boundary region of the image projected, it is possible to use low quality optics in the projection system, since any blurring of the projected boundary region will be compensated for by the method of detecting. The use of low quality optics while achieving the necessary accuracy permits the fabrication of an exceptionally low cost projector. Visual use is also possible at lower accuracy.
It is therefore an object of this invention to provide an improved wide angle projector for projecting an image having a boundary region over a wide angle.
It is another object of this invention to provide a projector for projecting an image having a boundary region in which cylindrical reflecting optics are utilized.
It is another object of this invention to provide a low cost, mechanically stable, wide angle projector utilizing cylindrical reflecting optics, reflective side walls, and a knife edge reticle for establishing the boundary region.
It is a yet still further object of this invention to provide a number of different reticles for the subject wide angle projector.
It is a still further object of this invention to provide a projector which utilizes a non-obscurring reticle and cylindrical reflecting optics having a parabolic or circular cross section.
It is a yet still further object of this invention to provide a color coding method and apparatus for eliminating any visual ambiguity between the correct boundary region and an incorrect boundary region.